Defects in current return paths are a common type of anomaly, which are in general difficult to localize in electrical assemblies. Thus, in state-of-the-art airplanes with skins made of composites such as carbon fiber reinforced plastics (CFRCs), the skin can no longer provide the current return path between the passengers and power supplies or provide electromagnetic protection, which functions were in the past provided by the metal skin of the airplane.
To overcome the above, networks of electrical structures are used. These networks incorporate primary metal structural parts and specific conductors. In particular, U-shaped or I-shaped metal structures (or “raceways”) are used to support cables along the length of the fuselage or cargo-hold ceiling of an airplane. These raceways consist of elements interconnected by metal braids, and are used both as screens to provide electromagnetic protection, and to support electrical cabling routed over large distances.
However, above all, these raceways provide a return current path for the currents carried by the cables, since they form a low-impedance current line with the cables that they contain. Under these conditions, it is essential to be able to detect the loss of a braid or the degradation of its electrical functions throughout the life cycle of the airplane.
Specifically, the cabling of modern airplanes can be several hundred kilometers in length and reliable diagnosis of the validity of the electrical network is something that is important, even vital.
This detection cannot generally be done with an ohmmeter because the raceways are connected to the ESN at many points—the loss or degradation of a braid is insignificant relative to the total resistance of the ESN. Individual testing of the braids is not possible for reasons of cost, due to their large number and inaccessibility.
Moreover, it has already been envisioned to use reflectometry, a diagnosis method based on the radar principle, to test for defects in electrical circuits. Reflectometry is a diagnosis method based on injecting a test signal into a medium to be diagnosed. Some of the energy of this signal, which propagates according to the propagation law of the medium in question, is reflected back toward the injection point when it encounters a discontinuity. The main advantage of this technique is that only a single circuit access point is required.
For example, patent U.S. Pat. No. 7,215,126 describes a mixed signal reflectometer in which a combined signal is generated comprising a superposition of a test signal injected at a single injection point, and reflections of this signal generated in the circuit. This reflectometer comprises a test signal generator, a detector configured to determine an autocorrelation of the combined signal, and an analyzer configured to evaluate a characteristic of the circuit depending on this autocorrelation.
This solution can be classed among the various known techniques for testing electrical cabling by reflectometry. These techniques comprise generating a test signal, detecting reflected signals, and analyzing these signals, like in the aforementioned apparatus. These techniques either function in the frequency domain (FDR) or in the time domain (TDR) depending on the nature of the injected signal: FDR uses a frequency-modulated sinusoidal signal, and TDR a modulated pulse signal.
Reflectometry may be used to test a network of raceways by coupling this network to electrical cabling composed of insulated cables and placed along the raceways. Each cable is then electrically connected at one of its ends to the associated raceway, and is equipped at its other end with a connector allowing it to be connected to a detecting and analyzing reflectometry device. A transmission line for transmitting the signal is thus created by the presence of cables associated with the ground plane formed by the raceway. Injecting a test signal into the cable via the connector results in a reflected signal, a timing diagram of intensity variations as a function of time.
In the graph obtained via an oscilloscope, using appropriate units, a first “positive” peak appears in the reflected signal, this first peak having a much larger amplitude than that of the other positive peaks: this first peak corresponds to the signal injected at the start of the line. The last “negative” peak (i.e. this peak is inverted relative to the first) corresponds to the signal reflected by the short circuit at the end of the line. Intermediate peaks, which appear between the first and last peaks are due to fluctuations in the impedance of the transmission line. The spectrum obtained is the reference signature of the raceway.
A raceway connection defect, for example disconnection or poor connection of a braid, generates an impedance break. The test signal will reflect from this break. This reflection results in a region of the graph deviating from the reference graph, a reflection peak forming in this region.
Such changes in the spectrum are detected, and the defects thus localized, during maintenance testing, by comparing the actual measurement and the reference signature recorded on delivery.
However, when the defects result in variations in the reflected signal with intensities that are equivalent to or about the same as those of the fluctuations in impedance, it becomes difficult to detect and localize this type of defect. However, the intensity of these fluctuations may be sufficiently large that certain defects go undetected, or become hard to detect.